Sarina, a married woman living in an apartment complex, is worried about her sex life with her husband, who has little stamina or libido. A middle-aged man named Kishikawa, who has just moved in next door, comes up with the idea of recommending her a tonic made by the pharmaceutical company where he works. A few days later, he visits Sarina's house and peeks inside through an open window, only to be greeted by the sight of her indulging in a lewd act, running her hands over her sweaty skin as she masturbates. The two, who were never meant to have sex, end up having an affair so intense that they end up covered in sweat after witnessing her masturbating...